The usual stop motions of this kind (see e.g. German OS No. 20 24 122) have a relatively long thread sensor bar and, thus, a relatively long distance between the pivot shaft and the thread guide eyelet so that, depending on the distance from the pivot shaft, the vibrations which are transmitted from the running thread to the thread sensor bar, are effective in an intensified degree in the range of this pivot shaft. The resulting increased load in the range of the pivot shaft leads to damages to the shaft bearings. This, in turn, is of negative influence on the sensitivity of response of the stop motion e.g. in case of a thread breakage. This may result in the fact that the thread sensor bar, upon the occurence of a thread breakage, is delayed in its pivotal downward movement under the force of gravity, from its operational position in which it is maintained by the running thread, to a lower position thereby releasing a signal or a control command for stopping the work station.
The vibrations which are transmitted from the running thread to the thread sensor bar can have an extremely negative effect in case of a two-for-one twisting spindle in which the stop motion forms the thread guide eyelet defining the apex of the thread balloon. The momenta originating from the thread balloon act in axial direction of the balloon as well as perpendicularly thereto. Depending on the length of the thread sensor bar, they are transmitted to the pivot shaft bearings in an intensified degree.